1.1 Energy stores and systems Flashcards
System
An object or group of objects.
When a system is in equilibrium, nothing changes so no energy is transferred.
When there is a change in a system, things happen so energy is transferred.
Thermodynamic systems
A thermodynamic system can be isolated closed or open.
An open system allows the exchange of energy and matter to or from its surroundings.
A closed system can exchange energy but not matter to or from its surroundings.
An isolated system does not allow the transfer of matter or energy to or from its surroundings.
Conservation of energy
Energy is stored in objects. When change occurs within a system, energy is transferred between objects or between stores.
Energy cannot be created or destroyed, it can only be transferred from one store to another. This means that for a closed system, the total amount of energy is constant.
8 energy stores
Kinetic, gravitational, elastic, magnetic, electrostatic, chemical, nuclear, thermal.
Energy transfer pathways
Energy is transferred between stores via transfer pathways.
Mechanical - when a force acts on an object.
Electrical - a charge moving through a potential difference.
Heating - energy is transferred from a hotter object to a colder one (conduction)
Radiation - energy transferred by electromagnetic waves.
Kinetic energy
The amount of energy an object has as a result of its mass and speed.
Ek = ½mv^2
Ek = kinetic energy in joules (J)
m = mass of the object in kilograms (kg)
v = speed of the object in metres per second (m/s)
Gravitational potential energy
The energy an object has due to its height in a gravitational field.
Eg = mgh
Eg = gravitational potential energy, in joules (J)
m = mass, in kilograms (kg)
g = gravitational field strength in newtons per kilogram (N/kg)
h = height in metres (m)
Elastic potential energy
The energy stored in an elastic object when work is done on the object.
Ee = ½ke^2
Ee = elastic potential energy in joules (J)
k = spring constant in newtons per metre (N/m)
e = extension in metres (m)
Thermal energy
Energy in the thermal store of an object is responsible for its temperature.
Energy can be transferred to or transferred from an object or system.
ΔE = mcΔθ
ΔE = change in energy, in joules (J)
m = mass, in kilograms (kg)
c = specific heat capacity, in joules per kilogram per degree Celsius (J/kg °C)
Δθ = change in temperature, in degrees Celsius (°C)
The amount of energy required to raise the temperature of 1 kg of a substance by 1 °C.
Practical 1 (specific heat capacity)
Aim: determine the specific heat capacity of a substance, by linking the amount of energy transferred to the substance with the rise in temperature of the substance.
Procedure - Start by assembling the apparatus, placing the heater into the top of the block.
Measure the initial temperature of the aluminium block from the thermometer.
Turn on the power supply and start the stopwatch.
Whilst the power supply is on, the heater will heat up the block. Take several periodic measurements.
Switch off the power supply, stop the stopwatch and leave the apparatus for about a minute. The temperature will still rise before it cools.
Monitor the thermometer and record the final temperature reached for the block.
Power
The rate of this energy transfer, or the rate of work done, is called power.
P = E/t or P = W/t
P = power in watts (W)
E = energy transferred in joules (J)
t = time in seconds (s)
W = work done in joules (J)
Conservation of energy
The law of conservation of energy states that:
Energy cannot be created or destroyed, it can only be transferred from one store to another.
This means the total amount of energy in a closed system remains constant
Energy can be transferred from store to store usefully (to do work).
Or energy can be dissipated to the thermal store of the surroundings.
Wasted energy
In practice, most systems tend to be open systems.
When energy transfers occur that are not useful, these are described as energy being dissipated to the surroundings.
This is considered to be wasted energy.
Often these less useful energy transfers often involve heating, light and sound.
Reducing unwanted energy transfers
Lubrication - Friction is a major cause of wasted energy in machines.
This wasted energy can be reduced if the amount of friction can be reduced.
This can be achieved by lubricating the parts that rub together.
Insulation - In many situations, the energy transferred by heating is wanted.
If this energy can be prevented from dissipating, then less energy will be needed to replace the wasted energy.
This can be achieved by surrounding the appliance with insulation.
The effectiveness of insulation depends upon:
How well the insulation conducts heat.
How thick the insulation is.
Efficiency
The efficiency of a system is a measure of the amount of wasted energy in an energy transfer.
The ratio of the useful energy output from a system to its total energy input.
Efficiency = (useful energy output/total energy input) x 100
Reducing friction
In a mechanical system, for example, there is often friction between the moving parts of the machinery.
This results in unwanted energy transfers by heating to the machinery and the surroundings.
Friction can be reduced by:
Adding bearings to prevent components from directly rubbing together.
Lubricating parts.
Reducing electrical resistance
In electric circuits, there is resistance as current flows through the wires and components.
This results in unwanted energy transfers by heating to the wires, components and the surroundings.
Resistance can be reduced by:
Using components with lower resistance.
Reducing the current.
Reducing air resistance
Air resistance causes a frictional force between the moving object and the air that opposes its motion.
This results in unwanted energy transfers by heating to the object and the surroundings.
Air resistance can be reduced by:
Streamlining the shapes of moving objects.
Practical 2 (investigating insulation)
Aim: investigate the effectiveness of different materials as thermal insulators and the factors that may affect the thermal insulation properties of a material.
Procedure - Set up the apparatus by placing a small beaker inside the larger beaker.
Fill the small beaker with boiling water from a kettle.
Place a piece of cardboard over the beakers as a lid. It should have a hole suitable for a thermometer and place the thermometer through this hole and into the water in the small beaker.
Record the temperature of the water in the small beaker and start the stopwatch.
Record the temperature of the water every 2 minutes for 20 minutes, or until the water reaches room temperature.
Repeat the experiment, each time changing the cardboard for another insulating material and also without any insulation at all.
